1. Field of the Invention
This invention relates generally to direct oxidation fuel cells, and more particularly, to components for managing fluids within such fuel cells.
2. Background Information
Fuel cells are devices in which electrochemical reactions are used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the nature of the fuel cell. Organic materials, such as methanol or natural gas, are attractive fuel choices due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel processing is complex, and requires expensive components, which occupy comparatively significant volume, the use of reformer based systems is presently limited to comparatively large, high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger scale applications. In direct oxidation fuel cells of interest here, a carbonaceous liquid fuel in an aqueous solution (typically aqueous methanol) is applied to the anode face of a membrane electrode assembly (MEA). The MEA contains a protonically conductive, but electronically non-conductive membrane (PCM). Typically, a catalyst, which enables direct oxidation of the fuel on the anode aspect of the PCM, is disposed on the surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). In the fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (from hydrogen in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the PCM, which is substantially impermeable to the electrons. The electrons travel through an external circuit, which includes the load, and are united with the protons and oxygen molecules in the cathodic reaction, thus providing electrical power from the fuel cell.
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, a mixture comprised predominantly of methanol and water is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the methanol and water in the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed at an acceptable rate (more specifically, slow oxidation of the fuel mixture will limit the cathodic generation of water, and vice versa).
Direct methanol fuel cells are being developed towards commercial production for use in portable electronic devices. Thus, the DMFC system, including the fuel cell and the other components should be fabricated using materials and processes that not only optimize the electricity-generating reactions, but which are also cost effective. Furthermore, the manufacturing process associated with a given system should not be prohibitive in terms of associated labor or manufacturing cost or difficulty.
Typical DMFC systems include a fuel source, fluid and effluent management and air management systems, and a direct methanol fuel cell (“fuel cell”). The fuel cell typically consists of a housing, hardware for current collection and fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing.
A typical MEA includes a centrally disposed, protonically conductive, electronically non-conductive membrane (“PCM”). One example of a commercially available PCM is NAFION® a registered trademark of E.I. Dupont de Nemours and Company, a cation exchange membrane comprised of polyperflourosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the liquid fuel mixture across the anode face of the PCM, while allowing the gaseous product of the reaction, typically carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to achieve a fast supply and even distribution of gaseous oxygen across the cathode face of the PCM, while minimizing or eliminating the collection of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM.
The diffusion layers are conventionally fabricated of carbon paper or a carbon cloth, typically with a thin, porous coating made of a mixture of carbon powder and TEFLON®. Such carbon paper or carbon cloth components allow a relatively high flux of methanol when immersed in a liquid methanol and water fuel mixture. While some methanol access through the anode diffusion layer is required for maintaining anode, and therefore, cell current, a high flux of methanol through the anode diffusion layer is a shortcoming because most presently available membrane electrolytes suitable for use in a DFMC system are typically permeable to methanol and concentrated fuel which, if introduced into the anode chamber, can thus pass at a significant rate through the diffusion layer and the membrane and oxidize on the cathode face of the membrane. This results in wasted fuel as well as diminished cathode performance, leading to diminished performance of the fuel cell and fuel cell system.
Traditional DMFC structures have required that the diffusion layers perform a current conduction function as well as managing the introduction and removal of reactants and products within the MEA. Thus, these layers have had to be electrically conductive, as well as capable of managing the transport of liquids and gasses within the MEA, i.e., transport reactants to and products away from the catalyst coated PCM. Diffusion layers used in fuel cells are comprised of porous carbon paper or carbon cloth, typically between 100-500 microns thick. Each of these diffusion layers is typically “wet-proofed” with TEFLON® or otherwise treated in a manner that makes the diffusion layer hydrophobic to prevent liquid water from saturating the diffusion layer. Such “wet-proofing” may not be ideal for the anode of a DMFC or other direct oxidation fuel cell system.
A metallic diffusion layer or a metallic diffusion layer combined with a flow field plate in a direct oxidation fuel cell has been described for use as a controlled methanol transport barrier. The metallic layer component can be manufactured using particle diffusion bonding techniques as described in commonly owned U.S. patent application Ser. No. 09/882,699 which was filed on Jun. 15, 2001, for a METALLIC LAYER COMPONENT FOR USE IN A DIRECT OXIDATION FUEL CELL.
Those skilled in the art will recognize that materials other than metals may offer advantages for certain architectures or designs. For example, many polymers are less expensive, and easier to mold or form into a desired structure than metals, provided that there are alternate structures and methods in place to collect current and provide other desired characteristics. In addition, the use of polymers allow for precise engineering of the size and shape of the pores in the component, and may be further desirable as it is possible to utilize a liquid impermeable polymer.
As noted, the MEA is formed of a PCM to which a catalyst is applied, forming a catalyst coated membrane (CCM). Diffusion layers are pressed onto the CCM. Generally, the entire MEA is held in place by a frame comprised of a faceplate that is disposed on each of the anode side and the cathode side. The faceplates provide an electron path while also providing compression to the MEA. The faceplates also physically connect the MEA to the fuel cell system. It is common to make the diffusion layers and catalyst layers the same dimensions as the openings of the frame. Fluid leaks are resisted by gaskets that are placed between the faceplates, around the MEA. However, this is not always successful, as there is an another path, around the diffusion layers, and across the catalyst layer, through which fuel may pass or leak from the anode to the cathode side of the PCM. This fuel crosses over the membrane, resulting in methanol cross over. The fuel is thus wasted as it does not contribute to the electricity generating reactions. Instead, it oxidizes on the cathode aspect of the MEA, thus creating heat. Excess heat can result in changes in the behavior of the NAFION® membrane, thus reducing fuel cell efficiency. Alternatively, the fuel can simply remain on the cathode aspect of the PCM also reducing fuel cell efficiency.
There remains a need, therefore, for an improved membrane electrode assembly in which undesirable leaks across or around the diffusion layers and/or the catalyst or other components are eliminated, without adding unnecessary bulk and weight to the fuel cell, and fuel cell system.
It is thus an object of the present invention to enhance fuel cell efficiency by eliminating possible paths for fluid to leak from the anode to the cathode of an MEA.